June 15, 2004

Phoebe

Images of Phoebe, a moon of Saturn, support the idea it is a captured comet.

The new pictures show that most of the moon is dark, but impacts have blasted holes in the surface to reveal much brighter material underneath, which is probably a mixture of ices. So Phoebe looks like a dirty snowball - the term coined to describe comets.

NASA has more on the Cassini-Huygens mission on-line including an animation of Phoebe here. Next up: Saturn by the first of July and an astonishing passage as the spacecraft crosses through the large gap between the F Ring and G Ring.

The main rings--D, C, B, A, and F--are quite bright and incredibly thin, no more than 10-30 meters thick compared to their expanse of 275 thousand kilometers across. (If the A ring was as wide as a football field, it would be as thick as a piece of tissue paper.)

Because the rings are bright, they must be made of chunks of icy material; because the rings are so thin, the chunks must be small, no more than a couple of meters across at most, and frequent collisions between ring particles would tend to break big chunks into smaller ones. In fact, little moonlets may constantly form and be broken apart. Cassini should be able to watch this happen.

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Comments

Gratuitous spam alert: I wrote a big-ass entry on my blog on the history of US planetary exploration, particularly about how Cassini is sort of the coelacanth of NASA probes, literally a "blast from the past".

But yah, fun stuff happening at NASA. Good to see unmanned planetary missions on the upswing again: I wasn't even born in the last great era of planetary exploration.